I have a 327 that will probably be pushing around 350 HP. The rear is a 3:08 posi.

Any recommendations as to which trans would work the best?

M20. It has a lower first gear and would work better with your 3.08s. A M20 has a 2.52 first gear and a M21 has a 2.20 first gear. M21s are better suited with 3.73 or numerically higher ratios, where M20s are better suited for 3.73 and numerically lower ratios. I have an M20/3.73 setup in my 67 and love it.

Great advice, thanks